The formation of iodinated trihalomethanes (I-THMs) from iodide and iopamidol, two major iodine sources to form iodinated disinfection by-products (I-DBPs) during monochloramination, was compared in the presence of different organic precursors including algae organic matter (AOM), Suwannee River humic acid (HA), fulvic acid (FA) and 2 raw waters. Effect of bromide, reaction time and pH on I-THM formation was also studied. The results showed that in the presence of HA and FA, monochloramination of iodide exhibited much higher I-THM formation than that of iopamidol, while in the presence of AOM, monochloramination of iopamidol formed more I-THMs than that of iodide. The presence of bromide increased I-THM formation from iodide with iodoform as the dominant species, while bromine-containing I-THM species increased significantly from iopamidol with bromochloroiodomethane as the predominant species. The reaction between monochloramine and iopamidol is relatively slow and long-lasting compared to that between monochloramine and iodide. I-THM formation from iodide in the presence of HA was maximum at pH 7 but significantly inhibited at pH higher than 7.5 from iopamidol in the presence of AOM. The correlation between the ratio of I-THMs to dissolved organic carbon concentration and the specific ultraviolet absorbance value of raw waters was positive for iodide but negative for iopamidol during monochloramination. (C) 2014 Elsevier B.V. All rights reserved.